1. Field of the Invention
The present invention relates to an internal diameter measuring method for a transparent tube.
2. Description of Related Art
Optical measuring apparatuses are used in order to conduct non-contact measurement of an external dimension of a measured object (such as an outer diameter of a cylindrical object). For example, a laser scan micrometer, image sensor micrometer, or light-section type 2D (two dimensional) shape measuring sensor is used. These apparatuses detect the outer diameter and the like of the measured object from measurements of shaded sections blocked by the measured object, using a plurality of parallel laser light beams arranged in a band shape or a laser beam scanning in a similar shape (see, for example Japanese Patent Laid-open Publication No. 2001-108413).
A measuring apparatus using such parallel laser light beams detects a change in an amount of light in a width direction of the parallel light beams when measuring the outer diameter and the like of the measured object. In other words, when the cylindrical object (as the measured object) is placed in the middle of a path of the parallel laser light beam, a middle portion of the light beams is blocked with the cylindrical object and only the light beams passing through two outer sides of the cylindrical object reach a photoreceiver. Specifically, an amount of received light detected by the photoreceiver is larger at the two outer sides of the cylindrical object but smaller at a shaded portion of the cylindrical object. Therefore, by detecting two positions on two sides in the width direction of the cylindrical object where the amount of received light rapidly decreases, the outer diameter of the cylindrical object can be measured from the distance between the two positions.
Further, in the measuring apparatus noted above, in order to detect the positions where the amount of received light rapidly decreases, a comparison is performed between a detection signal of the amount of received light and a predetermined threshold value. Specifically, there is a transition from a state where the amount of received light is large on the outer sides of the cylindrical object to a state where the amount of received light is small at the shaded portion of the cylindrical object. As a result, in the photoreceiver, it is possible to detect a first outside position of the cylindrical object when a photoreception signal decreases and is below the threshold value. In addition, there is a transition from a state where the amount of received light is small in the shaded portion of the cylindrical object to a state where the amount of received light is large outside of the cylindrical object. Accordingly, in the photoreceiver, it is possible to detect a second outside position on the opposite side of the cylindrical object when the photoreception signal increases and exceeds the threshold value.
The measuring apparatus using the parallel laser light beam noted above is used not only for the outer diameter measurement of the cylindrical object, but is suggested for use in an internal diameter measurement of a transparent tube (see, Japanese Patent Laid-open Publication No. H03-162606). In Japanese Patent Laid-open Publication No. H03-162606, among parallel laser beams passing through the transparent tube when the transparent tube is emitted, attention is paid to a specific beam in which a direction of an optical axis does not change and the internal diameter of the transparent tube (the maximum outer diameter of an inner circumferential surface) is geometrically calculated.
In FIG. 4, when a transparent tube 80 is placed on an optical path of a parallel laser light beam 90, the parallel laser light beam 90 is transmitted as is in regions E1 and E2 outside beams 91 and 92, which are tangent lines of an outer circumferential surface 81 of the transparent tube 80. In contrast, in a region E3 inside the beams 91 and 92, the parallel laser light beam 90 is blocked and a shade is formed. In the existing measuring apparatus, an outer diameter Do of the transparent tube 80 (the maximum outer diameter of the outer circumferential surface 81) is measured by detecting the positions of the beams 91 and 92. Further, the width direction of the parallel laser light beam 90 is a direction WD and an optical axis direction in which the parallel laser light beam 90 is emitted in a direction PD.
The parallel laser light beam 90 emitted toward the transparent tube 80 enters inside the transparent tube 80 refracted by the outer circumferential surface 81 of the transparent tube 80, is reflected by the inner circumferential surface 82, and is emitted from the outer circumferential surface 81 back to the outside. A portion of emitted beams 93 is in a state where the optical axis is parallel to the original parallel laser light beam 90. In FIG. 5, mainly three beams 94, 95, and 96 are observed in the shaded portion of the region E3. The beam 94 passes through a center of the transparent tube 80 and proceeds on the original optical axis without refracting since the beam 94 penetrates the outer circumferential surface 81 and the inner circumferential surface 82 orthogonally.
On the other hand, the beams 95 and 96 are reflected by the inner circumferential surface 82 mentioned above, emitted to an exterior again from the outer circumferential surface 81, and meet a condition that the emission optical axes are aligned to the original optical axes. By detecting the positions of the beams 95 and 96 (position in the width direction of the parallel laser light beam 90), using a geometric calculation based on those positions, two end positions of the inner circumferential surface 82 can be detected and an internal diameter Di of the transparent tube 80 (the maximum outer diameter of the inner circumferential surface 82) can be measured.
In the measurements of the internal/outer diameter of the transparent tube 80 as exemplified in FIGS. 4 and 5, based on the amount of light of the parallel laser light beam 90 passing outside the transparent tube 80 and the beams 95 and 96 transmitted through the transparent tube 80, the respective width direction positions (the width direction positions of the parallel laser light beam 90) are detected. In FIG. 6, the photoreceiver of the measuring apparatus receives the parallel laser light beam 90 passing outside the transparent tube 80 and beams 95 and 96 transmitted through the transparent tube 80, and outputs a detection signal S indicating an amount of light L for each width direction position P. In addition, in a scanning method, a scanning time stamp may be used instead of the width direction position P.
As noted above, in the regions E1 and E2 outside the beams 91 and 92 which are tangent lines of the outer circumferential surface 81 of the transparent tube 80, the parallel laser light beam 90 is transmitted as is and is received by the photoreceiver. Therefore, in the regions E1 and E2, the detection signal S indicates high amounts of light L1 and L2 respectively. On the other hand, in the shaded portion of the region E3, mainly the three beams 94, 95, and 96 are emitted parallel to the original optical axis (parallel laser light beam 90) and received by the photoreceiver. Therefore, in the region E3, peaks S4, S5, and S6 corresponding to the beams 94, 95, and 96 appear in the detection signal S. The peak S4 corresponds to the beam 94 transmitted through the center of the transparent tube 80 without refraction and indicates a high amount of light L4 relative to the light amounts L1 and L2. The peaks S5 and S6 correspond to the beams 95 and 96 reflected by the inner circumferential surface 82 of the transparent tube 80 and indicate light amounts L5 and L6 much smaller than the light amounts L1 and L2.
The internal diameter measurement of the transparent tube 80 noted above can be achieved by detecting the width direction positions of the peaks S5 and S6 corresponding to the beams 95 and 96. Specifically, a threshold value T is defined which is at a level intersecting with the peaks S5 and S6. The width direction positions P5 and P6, where the detection signal S exceeds the threshold value T, are detected and the internal diameter Di can be measured by performing a geometric calculation from these positions or a distance Dd between these positions.
However, in the internal diameter measurement of the transparent tube 80 noted above, the peaks S5 and S6 used in the calculation of the internal diameter Di each have width rather than being a single point, and also have a slope on both rising sides. Because of this, errors are unavoidable when detecting the positions (P5 and P6) exceeding the threshold value T.
In the outer diameter measurement for example, an intersection of the detection signal S with the threshold value is detected in an interval where the light amount L1 changes to the light amount L3. This point (corresponding to the beam 91) is determined to be outside of the transparent tube 80. When a similar procedure is applied to the inner diameter measurement, the intersection of the detection signal S with the threshold value T is detected in an interval where the light amount L3 changes to the light amount L5 at the peak S5. This point is determined to be the position P5 (corresponding to the beam 95). However, as shown enlarged in a circular region in FIG. 6, the actual position P5 is in the center of the peak S5. When the threshold value T is a threshold value T1 for example, the position P5 is offset from a point P51 where the detection signal S intersects with the threshold value T1 and an error is generated. In addition, depending on whether the threshold value T is the threshold value T1 or a threshold value T2, the intersection point may be P51 or P52, and an error may be generated. Such an error source is similar for the peak S6.
In this case, a distance Dd1 (calculated from the point P51 and a point P61 where the threshold value T is the threshold value T1) does not match the correct distance Dd calculated from the original positions P5 and P6. Furthermore, when the threshold value T fluctuates and the threshold value T1 is changed to the threshold value T2, the distance Dd1 changes to a distance Dd2 calculated from the points P52 and P62. As a result, since the positions P5 and P6, or the distance Dd may include an error, the internal diameter Di of the transparent tube 80 cannot be calculated accurately.